This invention relates to the formation of optical diffraction gratings (surface relief patterns) on the surfaces of sheets, foils and finished articles made of metals. The invention also relates to the articles exhibiting diffraction patterns thereby produced and to methods of making tools for impressing diffraction gratings onto surfaces of the articles. More particularly, although not exclusively, the invention relates to the patterning of sheets, foils and finished articles made of aluminum or aluminum alloys, such as beverage cans, foils, sheets, and the like, by imparting optical diffraction effects.
Aluminum and aluminum alloys are used nowadays for the formation of an ever increasing number of finished articles, including containers, foil packages and the like, intended for sale to the public at large either as finished products in themselves, or as part of the packaging of other items, such as foods and beverages. Articles of this kind are normally required to have a decorative finish, e.g. an applied coating, a paint layer, a transfer, a patterned transfer, or the like, in order to make such articles more attractive, more noticeable and therefore more saleable. Decorative finishes are an important marketing tool in an increasingly competitive market place.
As an example, it is to be noted that beverage cans made of aluminum are manufactured in large numbers and are usually decorated by the application of a paper label or, more commonly, by direct painting, lacquering or printing, or by the application of a decal. While such decorating methods make it easy to apply a wide range of patterns, messages and logos, etc., to the outer surface of the can body, they have the disadvantage that they make the recycling of used beverage cans somewhat more difficult (since the paper, paints, inks, etc., have to be removed in some way to avoid contamination of the recycled metal or interference with the remelting process) and, more importantly, they have the potential for causing pollution because they require the use of solvents or other harmful materials. As a result, expensive solvent recycling and scrubbing systems are required, or new technology making use of water-based inks, paints or lacquers, or the use of very small amounts of conventional solvents, must be developed.
It has long been known that very appealing decorative effects may be obtained by creating a diffraction grating on a surface of an article. A diffraction grating is a surface relief pattern formed by a series of ridges and/or grooves formed on or in the article surface, the ridges and/or grooves having spacings (i.e. distances separating one from another) in the order or the wavelength of visible light (about 1 micron). When illuminated with diffuse white light, diffraction gratings create light interference effects and produce visible coloured patterns with hues and intensities that often change with viewing angle (e.g. rainbow-like patterns). Such patterns are extremely attractive and eye-catching, and they may be created without having to apply any foreign substance onto the article surface.
One way of providing the outer surface of an article with a diffraction grating is to emboss the pattern of ridges or grooves directly onto the article surface by means of an impressing tool such as a die, roller, or the like, the tool itself having a bearing surface provided with a mirror image of the desired diffraction grating (i.e. projecting ridges where there are intended to be recessed grooves, or vice versa). Embossing techniques are effective for applying diffraction or holographic patterns to relatively soft materials, such as plastics (e.g. for applying security features to credit cards and bank cards). The patterns are typically applied using a nickel shim master, supported on a roll. The resulting embossed diffraction gratings may then be metallized (aluminized) by known methods to produce bright diffractive patterns or holograms.
However, it is not as easy to emboss diffraction gratings directly onto articles having harder surfaces, e.g. metal surfaces, particularly when subsequent metal treating operations are required. As an example, the formation of aluminum beverage cans is considered. The cans are first formed by drawing a metal cup from sheet metal and then lengthening the sides of the cup by passing the cup through two or three successively smaller ironing rings. A lid is then applied to the can body to close the container. If a diffraction grating were to be embossed on the surface of the stock sheet metal, it would be eliminated by the surface smoothing action of the drawing and ironing stages. On the other hand, if the finished can were embossed with the diffraction pattern, this would require high embossing pressures because the drawing and ironing steps make the metal xe2x80x9chardxe2x80x9d and it would be difficult as a result to avoid distorting the cans, even if they were well supported. A proposed method of carrying out such an embossing procedure on a can body is described in U.S. Pat. No. 5,881,444 which issued on Mar. 16, 1999 to Aluminum Company of America, in which the can is supported by a mandrel and pressed against a hardened embossed tool. It is noted that the process requires sufficient pressure to cause transfer of the embossed pattern to the can surface, while the pressure should not be so high that the wall of the can is distorted or deformed. In the experience of the inventors of the present invention, these two requirements cannot be satisfied satisfactorily at the same time, and an efficient pattern transfer is generally accompanied by an unacceptable distortion of the metal substrate. Similarly, if the pressure is reduced to avoid distortion, poor pattern transfer occurs. Moreover, the embossing step would have to be carried out very quickly in order not to delay the production rate of the cans (a typical line speed for can production is about 300 cans per minute). These factors make the embossing procedure unattractive for can bodies.
On the other hand, if the metal is soft enough to make the application of a diffraction pattern relatively easy (e.g. if the metal is in the form of a thin foil), the pattern may be distorted during the impressing step because the thickness reduction that will occur will be accompanied by an elongation or spread of the substrate metal.
U.S. Pat. No. 4,725,111 to Weitzen et. al., which issued on Feb. 16, 1988 to American Bank Note Holographics, (and corresponding divisional U.S. Pat. No. 4,773,718 which issued on Sep. 27, 1988) discloses a process of the above-mentioned type of applying diffraction patterns and holograms directly onto surfaces of metal materials, such as household aluminum foil and beverage cans. This is achieved by passing a metal foil or plate between a heated contoured embossing roller and a cooled counter roller, thus embossing a xe2x80x9cnegativexe2x80x9d of the diffraction pattern provided on the embossing roller directly onto the metal foil or plate. When this procedure is applied to beverage cans, the embossing procedure is carried out as a final step on the formed can bodies. A wheel is provided with a number of cooled mandrels on which the can bodies are positioned and then, as the wheel rotates, it brings each can body in turn into contact with a heated embossing roller which compresses the sidewall of the can between the surface of the embossing roller and the mandrel. As will be appreciated, this procedure has the disadvantages mentioned above, i.e. that a potentially slow additional step must be carried out on a metal that has been hardened by drawing and ironing. The heating steps soften the metal to some extent, but slows down the overall processing speed. The process as taught relies on careful control of pressure such that it is sufficient to transfer an embossed pattern but not so high as to result in xe2x80x9ccalenderingxe2x80x9d of the material (calendering is a term normally associated with paper mills and is the polishing or smoothening of the surface which occurs as a result of sliding of the product against a roll). In the context of the Weitzen et al. invention, it results in the smearing of the embossed surface against the roll and the consequent reduction in quality of the transfer. The patents also teach the careful control of temperature to maintain the metal in a low yield strength condition during the embossing.
There is thus a need for a process of producing an interference pattern on a surface of an article, e.g. a beverage can body, a metal foil, or other article, in a rapid and efficient manner which is consistent with mass production techniques.
An object of the invention is to provide a process of creating a pattern on a surface of an article, e.g. a beverage can, a metal foil, or other article, without necessarily using inks, paints, lacquers, or other non-metallic materials.
Another object of the invention is to produce an optical diffraction grating on a surface of an article in a rapid and efficient manner.
Yet another object of the invention is to provide a process of creating a fabricating tool, such as a die, ironing ring, or embossing roll, suitable for applying a diffraction grating to a surface of an article made, for example, of metal.
Thus, in one form, the invention provides a process of producing an article having a surface provided with an optical diffraction grating such that the surface generates visible colors when illuminated with diffuse white light. The process involves producing the article from a starting material by one or more steps, including a step in which a diffraction relief pattern is impressed on a surface of the article by a tool provided with a bearing surface having a corresponding surface relief pattern of parallel lines that contacts the surface of the article under pressure. During this impressing action, the material is caused to slide in a direction of movement relative to the bearing surface of the tool, and the tool is orientated such that the lines of the relief pattern on the bearing surface of the tool extend parallel to the direction of movement of the article as the diffraction relief pattern is impressed on the surface of the article.
During the impression of the diffraction grating, particularly to a sheet or foil article, the thickness of the article should preferably be reduced by at least 2%, and more preferably by at least 5%, to ensure the required relative movement between the surface of the article and the tool.
The invention also relates to articles, equally beverage cans, provided with color-generating diffraction gratings, produced by the process of the invention. However, the invention may also be used for decorating other types of containers and packages that may benefit from a more attractive appearance, e.g. food cans, aerosols, cosmetics containers, and the like.
The invention makes it possible to apply a diffraction grating to an article during a normal fabrication step, e.g. a rolling step during the production of a foil, or a final shaping step for a shaped article such as a beverage can, without creating delays in, or undue modification of, the manufacturing process. This is possible because the invention makes it possible to impress the pattern on a surface of the article without requiring an additional processing step.
Ironing is one process which can be used for creating the patterns of the invention in harder materials. xe2x80x9cIroningxe2x80x9d, for articles such as beverage cans, involves pushing a can body mounted on a mandrel through a ring die having a size which causes the metal to be thinned and elongated as it passes through the die. The can surface slides against the die surface as it is being ironed. Marks on the ring die are transferred to the can surface as score marks and are linear and parallel to the direction of sliding.
Another process used in the invention for producing diffraction patterns is extrusion. xe2x80x9cExtrusionxe2x80x9d resembles ironing in that it involves a change in cross-section of the material as it is passed through a die. However, the material is being forced through the die rather than being pulled by the mandrel and the final surface profile, which need not be circular, is controlled by the die surface. Marks on the bearing surface of the die produce elongated score marks on the surface of the article and are continuous over the length of the extruded article.
Yet another procedure which can be employed in the process of the invention is drawing or tube sinking. These techniques are similar in principle. xe2x80x9cDrawingxe2x80x9d involves pulling a material through a die rather than pushing it through as in extruding. xe2x80x9cTube sinkingxe2x80x9d involves pulling a tubular article through a smaller die to reduce the outer and inner diameters of the tube. Once again, both techniques involve moving one surface past another so that marks on the bearing surface create parallel scores or scratches on the article.
Yet another procedure that can be employed in the process of the invention is rolling. In this case, an elongated strip of material is passed between two rolls of a rolling mill, at least one of which has a relief pattern in the form of a circumferentially oriented diffraction grating across all or part of its width. As the strip passes through the roll gap, it is reduced in thickness and caused to elongate in the rolling direction. Within the region of contact, the diffraction grating pattern is impressed into the surface of the strip. As the strip is reduced in thickness and caused to elongate, it is forced to slide forwards relative to the roll surfaces (referred to as xe2x80x9cforward slipxe2x80x9d). Since the grating lines are parallel to the rolling direction, the relative slip between the sheet and the roll causes the grating pattern to be maintained or even enhanced as the strip exits the roll bite. It will be noted that grating lines in any direction other than substantially parallel to the rolling direction would be degraded or eliminated by the action of the forward slip. This process may be applied to strips or to foils. Foils tend to have a thickness in the range of 5 to 150 microns, and strips tend to have greater thicknesses.
In the present invention, calendering is not a concern, since any sliding of the metal against the roll will be parallel to the embossed pattern and will therefore not cause xe2x80x9csmearingxe2x80x9d and may even enhance the grating pattern.
It is to be noted that the impression of the diffraction grating in the process of the present invention may be carried out at ambient temperature (without external heating of the article) or at an elevated temperature, as desired. Generally, the diffraction grating may be applied during a typical xe2x80x9ccold rollingxe2x80x9d operation at temperature in the range of ambient to about 100xc2x0 C.